A fluid pump device of the type with which the present invention may be contrasted is disclosed in Australian Patent Application No. 57818/80, filed Apr. 24 1980. Such pump device is intended for use in conjunction with a pumping chamber which is composed of an elastic material and which has a pair of spaced-apart unidirectional valves through which fluid is induced to pass with cyclic compression and expansion of the chamber. The pumping chamber is located between a pair of clamp elements which are moved away from and toward one another with energization and de-energisation respectively of a solenoid to which a moving one of the clamp elements is coupled. Thus, when the solenoid is energised the pumping chamber expands toward its normal full volume, whereby fluid is drawn into the chamber from a fluid source, and when the solenoid is de-energised the pumping chamber is compressed so that it expels the fluid along a delivery line toward a recipient. Compression of the chamber is effected under the influence of a spring which biases the solenoid armature toward an open condition.
When the solenoid is de-energised and the armature is fully open, an electric circuit is closed to establish a time duration T which elapses between de-energisation of the solenoid and the armature reaching a fully open condition. The time duration T may be related either to the instant of de-energisation of the solenoid or, assuming a constant solenoid closing-time period, to the instant of energisation of the solenoid.
Ignoring for the time being various complications that will shortly be discussed, if the time duration T is less than a first predetermined time period T.sub.1 then indication is given, for example, that air is present in the fluid delivery line. Conversely, if the time duration T is greater than a second predetermined time period T.sub.2 then indication is given, for example, that a blockage is present in the fluid delivery line. Thus, detection is made for compression of the pumping chamber occurring "too quickly" or "too slowly".
However, the time duration T is determined to a large extent by the force exerted by the armature biasing spring and, hence, a high spring force will result in a low duration T and a low spring force will result in a high duration T. Therefore, if the spring force is set sufficiently high, the solenoid armature may be caused to open in time T less than T.sub.1 following de-energisation of the solenoid and an erroneous indication may be given that air is in the fluid delivery line. Conversely, if the spring force is sufficiently low, the solenoid armature may be caused to open in time T greater than T.sub.2 following de-energisation of the solenoid and an erroneous indication may be given that a blockage exists in the line.
If a simplistic approach were to be taken, the pump device might be constructed so that the spring force is maintained constant and is not adjustably variable. Then, the time duration T would vary solely as a function of the pressure in the fluid delivery line with the spring force being a constant factor.
However, such a simplistic approach is not feasible because the pressure in the delivery line will vary with a pressure differential between the source of the fluid and the fluid delivery point, for example, the injection point in a patient. Thus, different pressure differentials will occur according to the height of the fluid source above or below a patient and in dependence upon whether the fluid is to be injected into a vein or an artery.
Then, assuming that the time periods T.sub.1 and T.sub.2 are fixed as design parameters of the device, the duration T must be selectively variable to accommodate various possible pressure differentials, and the only way in which the time duration T can be varied independently of hydrostatic conditions is by making the spring force selectively adjustable.
Thus, the terminal end of the time duration T must be placed in a time slot which is relevant to the prevailing pressure differential between the source and delivery points of the pumped fluid AND which is located between the time periods T.sub.1 and T.sub.2 WHEN normal operating conditions exist. This can be accommodated by providing for manual adjustment of the spring force, as in the case of the device described in the above referenced patent application. However, manual adjustment of the spring force places a heavy onus on the user or operator of the pump device (for example, medical staff in the case of use of the pump device as an infusion pump), and it is desirable that the pump device incorporate a mechanism which is self adjusting as to the location of the time slot in which the time duration T is located according to the pressure differential that exists between the fluid source and the delivery point under normal operating conditions of the device. This is the central issue of the present invention.